Charophytes from the Pozo D-129 Formation (lower Aptian, Lower Cretaceous), source rock in the Golfo San Jorge Basin, Patagonia, Argentina

Abstract

The charophytes studied here are from the uppermost part of the Pozo D-129 Formation in Sierra Silva, which belongs to the Golfo San Jorge Basin. Fructifications from three charophyte species belonging to the families Characeae and Clavatoraceae are described and illustrated. Two charophyte assemblages obtained from the lower part of the section were characterized: Assemblage 1, formed exclusively of characeans, Mesochara stipitata and Lamprothamnium? barcinensis, and Assemblage 2, composed of L.? barcinensis and the clavatoraceans Clavator harrisii var. harrisii and Clavator harrisii var. zavialensis. These differences in the composition of the charophyte floras are related to different paleoecological conditions that changed from the temporary, unstable water bodies represented in the lower assemblage studied to more stable and permanent lake conditions. The charophyte assemblages together with the previously recorded palynological data suggest a late early Aptian age for the succession of the Pozo D-129 Formation studied. Likewise, the charophytes contribute to the paleobiogeographic history of two conspicuous taxa, M. stipitata and C. harrisii var. zavialensis, which are reported here for the first time in the Lower Cretaceous deposits of the basin.